Before I begin, I should admit that the title of this post probably oversells the depth and breadth of the content that follows. In fact, this post is going to focus exclusively on the one breast-related issue on which I may be considered an expert, and that is the relationship between breast fat and metabolic risk (if it sounds like I’m bragging, I’m not). I should also assure people that while this post does contain pictures, they are all completely safe for work.
As you can see in the above figure, fat tissue makes up a large proportion of the tissue – often the majority – within the breasts. And from what I understand, differences in breast size are due primarily to differences in the amount of fat tissue, as opposed to differences in duct or lobule volume. Similarly, changes in breast size due to weight loss (which can be seen in the photo at the bottom of this post) are due to reductions in the volume of fat within the breast.
Breast fat is a fascinating topic, but before we get to that specific fat depot, we need to briefly review the major types of fat within your body. If you’ve been following our blog for a few years this issue will be old hat, so feel free to skip to the next section.
All body fat is not created equal
In contrast to what some advertisements might have you believe, the fat in our bodies is not simply sandwiched between other tissues like a layer of butter on a baguette. Instead, fat is stored within specialized cells called “adipocytes” (hence fat is referred to as “adipose tissue”). Regardless of your body size, everyone has adipocytes in their body (as Peter has discussed in the past, having too few fat cells is actually much worse than having too many). When you lose or gain weight you are primarily changing the amount of fat stored in your adipocytes, rather than adding or removing actual fat cells.
The image below is taken from my Master’s thesis, which illustrates the three main adipocyte depots. The top image is a cross section of an abdomen, while the two bottom images are cross-sections of a pair of thighs.
Visceral adipocytes – these adipocytes are found within the abdominal wall and surround the internal organs (e.g. the viscera).
Intermuscular adipocytes – these are the fat cells that are found in between your muscles. The marbling on a steak is fat stored in these intermuscular adipocytes.
Subcutaneous adipocytes – this is the fat that you can pinch directly underneath your skin. Love handles, breasts, and anything mentioned in the song My Humps fall into this category.
The importance of this distinction is that these 3 fat depots have very different associations with health risk. Visceral and intermuscular adipocytes are generally very bad places to store body fat. Research has consistently shown that the more fat you store in these depots, the greater your risk of death and disease.
In contrast subcutaneous fat in the legs and buttocks actually seems to be protective against heath risk. For example, this paper from my MSc found that people with more subcutaneous fat in their lower body are actually healthier than people with the same body weight but less subcutaneous lower body fat. Despite being counter-intuitive, this is a very consistent finding (it made up the bulk of Peter’s PhD thesis). It’s also the reason why an “apple” body shape (e.g. lots of abdominal fat with very little lower body subcutaneous fat) is associated with much more health risk than the “pear shape”, characterized by fat stored mainly in the hips and thighs.
I’ve borrowed the two images below from one of Peter’s slideshare presentations, which nicely summarize the contrasting effects of visceral and subcutaneous fat accumulation.
There are a number of reasons why visceral fat stores are so much worse than lower body subcutaneous stores – as they expand visceral adipocytes become insulin resistant and promote inflammation, while also releasing fat into the blood stream where it can do damage to other tissues like the heart and liver. In contrast, subcutaneous adipocytes in the legs and butt tend to hold onto fat very tightly (hence why they are sometimes viewed as a “problem area” that are so difficult to slim down), which is actually quite beneficial from a health perspective.
What is the health impact of breast fat?
Peter and I first became interested in the health consequences of breast fat when we heard about this study published in the Canadian Medical Association Journal in 2008. Here is how Peter summarized the study in a previous post:
While it may sound like a ridiculous headline from The National Enquirer, a study published in 2008 , actually found that all else being equal – women with larger breasts had an increased risk of type-2 diabetes.In the study, published in the Canadian Medical Association Journal, Ray and colleagues assessed the prospective risk of developing type-2 diabetes according to bra cup size among a sample of over 92 000 women participating in the Nurses’ Health Study II.
After taking into consideration numerous established risk factors such as physical activity, smoking, diet, family history of diabetes, body mass index and waist circumference, among others, it was shown that in a graded fashion, the bigger the bra cup size – the greater the risk of developing diabetes. Specifically, in comparison to women with an A cup, women with a B, C, and D or greater bra cup had a 32, 71, and 58% greater risk of developing diabetes, respectively.
As I mentioned earlier, breast fat is a type of subcutaneous fat depot. Given what I’ve just said about subcutaneous fat, Peter and I found these results to be very surprising. Numerous studies, – including several by Peter, myself, and our former colleagues at Queen’s University – have shown that lower body subcutaneous fat stores are good for you. So breast fat should also be good for you, right?
The above study had a number of important limitations (asking women to recall their bra cup size from decades earlier surely isn’t the most accurate way to quantify breast size), but was generating a lot of media attention. And much of that attention was leading to understandable but troubling comments along the lines of
“I had been thinking about breast reduction before, but now that I know I can not only relieve back pain, but also reduce my risk of diabetes, I am definitely going through with the procedure.”
Enter the MRI scans
To be honest, Peter and I were surprised and a bit skeptical about these results, and especially the message that people were taking from the study. Fortunately, we felt we could address the question in a much more accurate way than the earlier study.
At the time Peter and I were working with Dr Bob Ross, who is an expert on the relationship between various fat depots and measures of health. Much of the work in his lab involves using MRI and CT scanners to take 40+ cross-sectional images of a person’s body. The different tissues (bone, muscle, subcutaneous fat, visceral fat, intermuscular fat, etc) in these images are then painstakingly coloured in one at a time by members of Dr Ross’ lab, and then used to calculate the volume of each tissue (assessing the various fat depots for just 1 person takes almost a full day). Angela Alberga, a colleague of mine here in Ottawa has spent the better part of the last 4 years analyzing MRIs for one large exercise study – it’s a lot of work! However, the result is an extremely accurate assessment of a person’s total body fat, as well as the amount of fat that they store in each depot (the only other method that rivals the accuracy of MRI and CT for assessing total fat is to put a body in a blender and extract the fat using ether, but this has certain obvious drawbacks).
The images below shows how these 40+ separate images can be used to create a detailed picture of a person’s fat stores (each colour represents a different type of tissue).
Through the course of his research Dr Ross had built up a large database of MRI scans, but no one had ever thought to look specifically at adipose tissue in the breasts. So Peter and developed a standardized protocol, and re-analyzed the scans of 92 pre-menopausal women to specifically quantify the volume of breast fat for each participant.
The image below comes from the resulting paper, which shows how we quantified breast adipose tissue on each image.
Somewhat surprising results
So what did we find? Breast size in our study was not associated with any marker of metabolic risk. In other words, a woman’s breast size didn’t seem to tell us anything good or bad about her risk for diabetes or cardiovascular disease. That could be taken to somewhat support our view that breast fat shouldn’t have a health impact. However, our findings weren’t so clear cut.
You see normally when you compare two people with the same body weight, the person with more lower body subcutaneous fat will tend to have less visceral fat. That’s one of the reasons why lower body fat is protective – if you’re storing fat there, you’re not storing it viscerally. However, that seems not to be the case for breast fat.
After controlling for age, BMI, and waist circumference level, breast volume remained positively associated with VAT and intermuscular fat (IMAT) (P < 0.05), such that women with the highest breast volume had ~1.1 and 1.3 kg more VAT and (IMAT), respectively, but no more abdominal or lower-body SAT, by comparison to women with the smallest breast volume. Thus, the previously documented association between breast size and type 2 diabetes risk may be in part explained by excess VAT and/or IMAT deposition.
Contrary to what we expected, it appears that all subcutaneous fat may not be created equal.
New study supports previous findings
Our results have been supported by a recent paper in the European Journal of Clinical Nutrition. Like us, the authors calculated breast adipose tissue on multiple images, then combined them to calculate total breast size. The picture below from their paper gives a nice idea of how this is done (left column is original baseline image, middle column is baseline image with breast adipose tissue highlighted, right column is scan of same person following a weight loss intervention).
Although not identical, their results were similar to our own:
A high proportion of [breast adipose tissue] was associated with higher truncal and lower leg [subcutaneous fat].
No relationships were found between [breast adipose tissue] and cardiometabolic risk factors. By contrast, [trunk subcutaneous fat] and [visceral fat] showed positive and [leg subcutaneous fat] inverse associations with cardiometabolic risk factors in cross-sectional as well as longitudinal analysis. The association between BrAT and VAT was lost after adjusting for %FM and truncal SAT.
What’s the take-home message?
On its own, breast fat doesn’t seem to tell us much about a woman’s metabolic risk profile. However it seems that for a given body size, women with larger breasts tend to store more visceral fat and less lower body subcutaneous fat, which could lead to increased health risk. In other words breast size really just seems to be an indicator for body fat distribution, rather than having any direct impact on health risk.
Does this research have any practical or clinical implications?
No it does not.
This research suggests that breast size may tell us a small amount about a woman’s tendency to store fat in either her hips or her abdomen. But we can already measure those fat stores directly by measuring waist, thigh and hip circumference. Plus there is no simple way to accurately assess breast size in a clinical setting. MRI scanners cost ~$400/hour, and it’s not easy to find an available scanner in most cities, while bra cup size doesn’t provide enough accuracy.
Also, keep in mind that there is absolutely no reason to think that increasing/reducing breast size will have a positive influence on metabolic health. Breast reduction surgery may benefit your back, but it’s not going to reduce your risk of diabetes. In fact, it may have the opposite effect, since you generally want to have more, rather than less adipocytes anyway.
In the end this is basic, albeit quirky, research aimed at better understanding the way that body fat distribution impacts health risk. It’s an interesting area, but it’s not going to affect the way that doctors tackle obesity-related health risk, and nor should it.
Janiszewski, PM, Saunders, TJ, & Ross R (2010). Breast Volume is an Independent Predictor of Visceral and Ectopic Fat in Premenopausal Women Obesity DOI: 10.1038/oby.2009.336